1. Field of the Invention
The present invention relates to a multiple-cylinder internal combustion engine provided with cylinders and having a cylinder head provided with a centralized exhaust passageway for collecting the exhaust gas discharged from combustion chambers corresponding to the cylinders.
2. Description of the Related Art
A multiple-cylinder internal combustion engine having a cylinder head provided with a centralized exhaust passageway and not having an exhaust manifold is disclosed in JP-A 2002-70551. In this known multiple-cylinder internal combustion engine, the exhaust gas discharged from the combustion chambers corresponding to the cylinders is collected in the centralized exhaust passageway formed in the cylinder head.
In the multiple-cylinder internal combustion engine having the cylinder head provided with such centralized exhaust passageway, the exhaust gas flowing through the centralized exhaust passageway can be efficiently cooled by using a water jacket having a large capacity for cooling walls defining the combustion chambers. Omission of an expensive exhaust manifold reduces the cost of the multiple-cylinder internal combustion engine. There is a growing tendency for the temperature of the exhaust gas to increase with the continuous increase in the output power of the internal combustion engine. Thus the reduction of the temperature of the exhaust gas is still important. Increase of the capacity of the water jacket may be a possible measure for efficiently cooling the exhaust gas. However, increase in the capacity of the water jacket involves increase in the size of the cylinder head. On the other hand, it is desired to avoid the reduction of the output of the internal combustion engine attributable to the improvement of cooling efficiency as far as possible.